A variety of monoclonal antibodies (hybridomas) have been recently developed that distinguish among classes of normal human lymphocytes and identify discrete stages of differentiation. In addition new monoclonal antibodies against human B cells are being developed in our laboratory. We are using a battery of these antibodies to determine the phenotypes of human malignant lymphomas using a Fluorescence Activated Cell Sorter (FACS-II). The phenotypic expression of these neoplastic lymphocytes is then related to normal lymphocytes and is useful in diagnosis and monitoring of patients' tumors during therapy.